The present invention relates to an improvement in a resonant converter which converts a direct current to an alternating current as an alternating current output by a semiconductor switching device, and supplies the alternating current output to a primary side of a transformer via a resonance circuit to convert an alternating current flowing through a secondary side of the transformer into a direct current.
A DC-DC converter has been widely used as an isolated power supply for the process input/output device of a computer control system or as a multioutput DC power supply for an information and communication equipment, electronic equipment and so on. One of types of such DC-DC converters is a resonant converter, which is disclosed, for example, in JP-A-2002-247854 or in “Multioutput Current-Mode Resonant Converter Using MAGAMP Control”, Switching Power Supply System Symposium 2002, Japan Management Association, (C6-2-1 to C6-2-10) (see C6-2-8 for its entirety and objects) (which will be referred to as Literature 1, hereinafter). That is, the resonant converter basically includes a main switching device (DC-AC converter), a resonance circuit, a transformer, and a rectifier (AC-DC converter). And a smoothly varying current is obtained by the resonance circuit, and a switching loss is reduced by the soft switching of the switching device in the vicinity of a zero voltage and a zero current thereof, thus realizing a high efficiency and low noise.